Structural health monitoring systems are attracting more and more interest. They provide a non-destructive method of evaluating the integrity of a structure. For example, use of structural health monitoring systems in the aerospace industry is becoming more widespread.
This is because such systems provide a non-invasive method of evaluating the health of an aircraft (or certain of its components) that is generally quicker and less expensive to perform than traditional counterparts. Moreover, the incorporation of structural health monitoring systems may allow longer intervals between service inspections, and may allow a lengthened service life of the aircraft.
Structural health monitoring systems have been used with both metal (e.g. aluminium and aluminium alloys) and composite structures.
Structural health monitoring systems comprising arrays of transducers, such as piezoelectric transducers, that monitor the propagation of elastic waves through a structure are known. Anomalies within the structure reflect the elastic waves, and these reflections are detected by the transducers. For example, delamination of resin-fibre composite structures and cracks in both composite and metal structures cause reflections that may be detected.
In addition merely to identifying the presence of one or more anomalies and or at least one anomaly, the location of an anomaly may be determined. For example, it is known to use a distributed array of transducers. Differences in times of arrival of the reflected wave at the transducers are converted to an equivalent distance from each transducer, from which the position of the anomaly may be determined using triangulation.
However, these systems are expensive and time consuming to install due to the large number of distributed sensors that must be provided and located. Furthermore, the large number of sensors results in a weight penalty for the aircraft, and also affects the basic parameters of the structure such as its inertia distribution and stiffness.
An alternative to a distributed array of transducers is to use a phased array of transducers. In such an array, the transducers are located in close proximity to one another. As is well understood in such applications as phased array antennas, the transducers are excited using an excitation signal whose phase varies between the transducers, thereby effecting beam steering. The transducers listen for reflected signals, and determine the location of one or more anomalies and or at least one anomaly by using the measured time of flight of the reflection and the known direction of propagation.
However, such phased-array systems also suffer from disadvantages. The beam-steering technique relied upon with phased arrays requires complex signal processing to extract directional information. Moreover, such systems suffer from signal leaks to the sides of the array such as grating lobes, along with strong secondary signals at angles other than the intended steering angle. Also, the required precision in beam forming and steering places exacting requirements on transducer specification and installation.
Hence, the established techniques of distributed arrays and phased arrays of transducers suffer from complexity, and tight manufacturing and design requirements, that increases significantly the expense of fabrication and integration within a structure. The complexity of the data generation and processing further increases cost.
Consequently, there is a need for a structural health monitoring system with reduced complexity, reduced cost and easier installation.